x INTRODUCTORY 
As to sub-orders, families and genera we shall to a great extent follow 
Gray’s ‘Catalogue,’ or H. and A. Adams’ exposition, in their ‘Genera,’ 
although we shall not refrain from making such alterations as appear called for by 
the examination of the fossil shells.* 
Until within the last few years, it has been the usual custom in fossil Con- 
chology to follow a few standard works, which had been carefully executed in 
accordance with the systems of Lamarck and Cuvier. There can be no question 
that this course very greatly facilitated the understanding of the general character 
of the shell, especially where the paleontologist had to deal with only a small 
number of species. Still a great drawback in fossil Conchology has been produced, 
in that the comparison of the extinct forms with living ones has been so much 
neglected, or at least retarded. A large number of generic names has thus been 
introduced into the literature of Conchology, very often for no other reason 
than that all the species belonging to them were extinct, or at least were believed 
to be so. Fossil shells also were occasionally treated solely with reference to their 
identity, similarity, or difference as compared with those already described from 
the same or similar deposits, and as being characteristic for a certain geological 
formation. It is only natural that the importance of the last pomt should not 
be overlooked by the paleontologist; but it was not until Ed. Forbes, Darwin, 
Dana, Lyell, and others showed the vast importance of applying results, derived 
from the study of physical conditions in connection with the recent fauna, to the 
examination of the fossil faunze, that the great physico-geological results, which 
we now see rapidly progressing, received that attention which was due to them. It 
was proved that species of a certain form and organization only live or prosper 
under certain favorable circumstances, at a certain depth, temperature, and other 
conditions of the water, or climate, on certain ground, &c., and that from an ex- 
amination and correct recognition of the characters of the fossil shells, it is possible to 
form correct conclusions as to the past physical conditions, under which the res- 
pective strata have been deposited. An enormous field of new and most interest- 
ing inquiry into the former conditions of life on our globe was thus opened for the 
geologist and palzeontologist. The ‘formations’ of the geologist ceased to be looked 
upon merely as a series of beds with some characteristic fossils, but appeared in the 
light of a series of beds deposited under various physical conditions and in a great 
ocean swarming with organic beings, which were adapted to those various conditions. 
* The want of references vastly increases the difficulty in the critical use of the book, and is, as well as 
the very strange and unusual names occasionally adopted, justly complained of by many conchologists. 
